Aluminum sheet is used on a wide scale in building structures as interior and/or exterior panels of buildings both for facade and roofing structures. An advantage of this is that the low specific weight of the aluminum means that the building structure may be made considerably lighter than for example with steel sheet. A disadvantage of untreated aluminum sheet is that the building structure reflects a large amount of light which limits its application in the immediate vicinity or airfields, for example. A solution to this problem is to provide the aluminum sheet with a surface layer, for example zinc, while retaining the structural advantages. An advantage of a galvanized aluminum sheet is that it reflects less light and has good corrosion resistance. A further advantage of galvanized aluminum sheet is that the building structure made with it requires little maintenance due to the durability of the AlZn system. A further advantage of galvanized aluminum sheet is that the appearance of the aluminum sheet changes slowly over time, i.e. it "lives". This last property, the so-called patina effect, is much sought after by architects for application in buildings of their design. Galvanizing the aluminum sheet considerably increases the applicability of the aluminum sheet.
Methods of electrolytically plating zinc onto aluminum (galvanizing) are well known. To improve the applied zinc layer, pretreatments of the aluminum surface have been proposed.
JP-A-52005630 discloses electroplating one of Cu, Ni, Zn, Sn, Pb, Cd and Cr onto the chemically roughened surface of an aluminum or aluminum workpiece. The roughening is performed in two stages: first in alkali pH.gtoreq.11 or a fluoric acid solution, and second using a mixed mineral acid solution.
In EP-A-0497302 a pretreatment step consisting of degreasing and pickling is performed and cathodic zinc electroplating is then carried out in two steps, thereby forming two adjacent zinc layers. The layers may contain Ni or Fe additions to improve bonding.
Another method for pretreating and galvanizing an aluminum sheet is given in EP-A-0498436, which describes a method for the continuous electrolytic application of a zinc layer onto an aluminum sheet intended for processing into an automotive body sheet, after which the aluminum sheet is provided with a paint layer. The method comprises in succession the steps (i) alkaline degreasing, (ii) pickling in an acid solution, (iii) anodizing and cathodic zinc electrolysis of the aluminum sheet in the same acid solution. In between the steps the aluminum sheet is cleaned by rinsing with water. The anodization and electrolysis process is not dependent on the extent of pretreatment, which may even be omitted. A disadvantage of galvanized aluminum sheet obtained by this method is that the bonding of the applied zinc layer on the aluminum sheet is very poor when the galvanized aluminum sheet is highly deformed, for example by bending.
Zinc plating of wrought aluminum sheet as a precursor to zinc phosphate treatment and painting is described in U.S. Pat. No. 5176963. The zinc plating may comprise two stages, namely displacement plating from an alkaline bath and electroplating from an acid bath. There is no disclosure of the application of a potential to the sheet during the displacement plating stage.